Hydraulic well stimulation operations such as those involved in fracturing geological formations adjacent deep well bores, erosion drilling, and the like present difficult problems due to the depth of the formation to be fractured, the high pressures required to be generated, the corrosive and abrasive nature of the fluids to be pumped, the long pumping times, and various other factors known to persons skilled in the art.
Conventionally, these operations have been performed by a series of mechanically geared or revolving crank pumps having relatively short strokes and relatively high cycles per minute; for example, eight inch strokes and 120 cycles per minute. Such pumps tend to fatigue and to break down rather readily when used for well stimulation, because of the extreme pressures and the high cycles per minute rate of operation. Such pumps also exhibit pressure pulsations or transient fluctuations which aggravate the adverse effects created by the high rate of fatigue and wear cycles. As a result, effective and profitable well stimulation was impeded.
One successful approach taken to meet this problem was disclosed in U.S. Pat. No. 3,773,438. The device disclosed there was a two cylinder intensifier which maximizes the volume per stroke and reduced the rate of operation by using large diameter pumping rams having relatively long strokes. The intensifier included two hydraulic ram assemblies, each composed of a working ram and a power ram or cylinder for driving the working ram on its forward pumping stroke. Separate return cylinders were provided to drive each ram assembly on its return stroke. Such return cylinders unnecessarily complicated the intensifier.
The hydraulic well stimulation apparatus disclosed in U.S. Pat. No. 3,773,438 also provided for precompression of the rams prior to each forward pumping stroke. This precompression allowed for a smooth transition between the end of the pumping stroke of one ram and the beginning of the pumping stroke of the other ram. Such precompression was accomplished by supplying the driving fluid directly to the ram assembly slightly prior to the initiation of its forward stroke. It was found, however, that pre-application of the driving fluid to accomplish precompression of one ram resulted in a drop in driving fluid pressure to the other ram which was finishing its pumping stroke. This of course resulted in a momentary decrease in the pressure output of the fluid being pumped. Another problem experienced during precompression was that the ram being precompressed might move slightly and deactivate a limit switch which was used to signal the control circuit that the ram was appropriately positioned for precompression.